Zoom lens and imaging device

ABSTRACT

Disclosed is a zoom lens with a reduced thickness and size in the depthwise direction.  
     The zoom lens of the present invention consists of a plurality of lens groups (GR 1  to GR 5 ), so that it varies in power in response to variation in intervals between the lens groups. The zoom lens also has a prism G 2  to bend the optical axis passing through the lens groups (GR 1  to GR 5 ). The last lens group G 5  (counted from the object side) is composed of a negative lens group and a positive lens group, with an air layer interposed between them (arranged sequentially from the object side). The present invention also provides an imaging device equipped with an imaging element to convert the optical images formed by said zoom lens into electrical signals.

TECHNICAL FIELD

The present invention relates to a zoom lens and an imaging deviceequipped therewith, said zoom lens being suitable for the imagingoptical system of digital input and output devices such as digital stillcameras and digital video cameras on account of its compact size andvariable power.

BACKGROUND ART

Recent years have witnessed the wide diffusion of the imaging devices,such as digital still cameras, which are equipped with a solid-stateimaging element. Digital still cameras are required to have an improvedimage quality as they become popular more than before. Particularly,those digital still cameras equipped with a solid-state imaging elementhaving a large number of pixels need an imaging lens, especially a zoomlens, with good image-forming performance. They are also required to besmall in size, and hence there is a strong demand for a small-sizehigh-performance zoom lens. (See Japanese Patent No. 2750775 [Patentdocument 1]) On the other hand, attempts are being made to reduce thesize of the zoom lens in the direction of optical axis by bending theoptical system with a prism inserted between lenses. (See JapanesePatent Laid-open No. 248318-1996 [Patent document 2])

Insertion of a prism is a very effective way to reduce the lens diameterand length (or to reduce the overall lens size) for the optical systemhaving the positive refracting power at the object side and the negativerefracting power at the image side, in the case of conventionallens-shutter cameras for silver salt film. Unfortunately, it does notpermit microlenses to fully exhibit their condensing performance becausemicrolenses have the exit pupil near the image surface and are arrangedin front of the solid-state imaging element. The problem is that theimage brightness extremely varies in going from the image center to theimage edge.

The object of miniaturization is not fully achieved in the opticalsystem equipped with a solid-state imaging element (which is disclosedin the patent document 1), because the optical system employs a negativelens group as the last lens group which is limited in power. The objectof miniaturization is not fully achieved either in the optical systemdisclosed in the patent document 2, which is designed to reduce the sizein the direction of the optical axis by bending the optical axis with aprism inserted in the positive-negative-positive-positive zoom type,because the optical system employs a front lens and a reflecting memberwhich are large in size.

DISCLOSURE OF THE INVENTION

The present invention was completed to address the above-mentionedproblems. Thus, the present invention is directed to a zoom lens and animaging device equipped therewith. The zoom lens is composed of aplurality of lens groups arranged at variable intervals and hence iscapable of power variation. Moreover, it contains a reflecting member tobend the optical axis, and its lens groups are characterized in that thelast lens group (counted from the object side) consists of a negativelens group and a positive lens group, with an air layer interposedbetween them, which are sequentially arranged from the object side.

In addition, the present invention is directed to a zoom lens and animaging device equipped therewith. The zoom lens is composed of aplurality of lens groups arranged at variable intervals and hence iscapable of power variation. Moreover, it has the lens groups which arecharacterized in that the last lens group (counted from the object side)has a negative refracting power and consists of a negative lens groupand a positive lens group, with an air layer interposed between them,which are sequentially arranged from the object side.

The advantage of the present invention is that the entire lens systemcan be miniaturized and the position of entrance pupil can be placedaway from the image plane. This leads to size reduction and thicknessreduction for the zoom lens and the imaging device equipped therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the lens arrangement of the zoom lens in thefirst example, which is adjusted to the end position of the short focallength.

FIG. 2 is a diagram showing the lens arrangement of the zoom lens in thesecond example, which is adjusted to the end position of the short focallength.

FIG. 3 is a diagram showing the lens arrangement of the zoom lens in thethird example, which is adjusted to the end position of the short focallength.

FIG. 4 is a diagram showing the lens arrangement of the zoom lens in thefourth example, which is adjusted to the end position of the short focallength.

FIGS. 5A to 5C are diagrams showing the aberrations which the zoom lensin the first example experiences when adjusted to the end position ofthe short focal length.

FIGS. 6A to 6C are diagrams showing the aberrations which the zoom lensin the first example experiences when adjusted to a position of theintermediate focal length.

FIGS. 7A to 7C are diagrams showing the aberrations which the zoom lensin the first example experiences when adjusted to the end position ofthe long focal length.

FIGS. 8A to 8C are diagrams showing the aberrations which the zoom lensin the second example experiences when adjusted to the end position ofthe short focal length.

FIGS. 9A to 9C are diagrams showing the aberrations which the zoom lensin the second example experiences when adjusted to a position of theintermediate focal length.

FIGS. 10A to 10C are diagrams showing the aberrations which the zoomlens in the second example experiences when adjusted to the end positionof the long focal length.

FIGS. 11A to 11C are diagrams showing the aberrations which the zoomlens in the third example experiences when adjusted to the end positionof the short focal length.

FIGS. 12A to 12C are diagrams showing the aberrations which the zoomlens in the third example experiences when adjusted to a position of theintermediate focal length.

FIGS. 13A to 13C are diagrams showing the aberrations which the zoomlens in the third example experiences when adjusted to the end positionof the long focal length.

FIGS. 14A to 14C are diagrams showing the aberrations which the zoomlens in the fourth example experiences when adjusted to the end positionof the short focal length.

FIGS. 15A to 15C are diagrams showing the aberrations which the zoomlens in the fourth example experiences when adjusted to a position ofthe intermediate focal length.

FIGS. 16A to 16C are diagrams showing the aberrations which the zoomlens in the fourth example experiences when adjusted to the end positionof the long focal length.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described in more detail with reference to itspreferred embodiments. The zoom lens demonstrated in the embodiments isa compact one intended for use with imaging devices such as videocameras and digital still cameras. The present invention covers a zoomlens of the type having a plurality of lens groups and varying in powerin response to variation in intervals between the lens groups, whichcomprises a reflecting member to bend the optical axis passing throughthe lens groups and a last lens group (counted from the object side)which is composed of a negative lens group and a positive lens group,with an air layer interposed between them (arranged sequentially fromthe object side). The present invention also covers an imaging deviceequipped with an imaging element which converts the optical image formedby the zoom lens into electrical signals.

In the zoom lens according to the present embodiment, the lens groupsshould preferably be constructed such that the first lens group (countedfrom the object side) is stationary and contains said reflecting member.Moreover, in the zoom lens according to the present embodiment, the lensgroups should preferably be constructed such that the last lens group(counted from the object side) has a negative refracting power.

In the zoom lens according to the present embodiment, the negative lensgroup of the last lens group should preferably satisfy the conditiondefined by the inequality (1) below.0.9<|fa/fw|<1.25where, fa denotes the focal length of the negative lens group in thelast lens group, and fw denotes the focal length at its wide end.

The inequality (1) given above defines the focal length of the negativelens group in the last lens group. If the focal length is smaller thanthe lower limit of the inequality (1), then it would be difficult tocorrect the edge coma and the chromatic aberration of magnification. Ifthe focal length is larger than the upper limit of the inequality (1),then the negative lens group has a weak power which preventsminiaturization.

The zoom lens according to the present invention may be composed of aplurality of lens groups alone, with the above-mentioned reflectingmember omitted. Incidentally, in the case where a prism is used as thereflecting member to bend the optical axis, it is desirable to selectone which is made of glass with a high refractive index.

EXAMPLES

A description is given below of the examples of the present invention.FIG. 1 is a diagram showing the lens arrangement of the zoom lens in thefirst example. The arrows in the figure represent the loci along whichthe lens groups move in going from the wide end position to the tele endposition. The zoom lens in the first example consists of a first lensgroup GR1 (positive), a second lens group GR2 (negative), a third lensgroup GR3 (positive), a fourth lens group GR4 (positive), and a fifthlens group GR5 (negative), which are arranged sequentially from theobject side. The first lens group GR1 consists of a negative lens G1, arectangular prism G2 to bend the optical axis through 90°, and apositive lens G3 having aspherical surfaces on both sides.

The second lens group GR2 consists of a negative lens G4, a negativelens G5, and a positive lens G6, which are cemented together. The thirdlens group GR3 is a positive lens G7 having aspherical surfaces on bothsides. The fourth lens group GR4 consists of a positive lens G8 havingan aspherical surface at the object side and a negative lens G9, whichare cemented together. The fifth lens group GR5 consists of a negativelens G10 and a positive lens G11, which are cemented together, and apositive lens G12. Incidentally, “LPF” denotes a filter, “CG” denotes acover glass, and “IMG” denotes the receiving surface of the imagingelement.

FIG. 2 is a diagram showing the lens arrangement of the zoom lens in thesecond example. The arrows in the figure represent the loci along whichthe lens groups move in going from the wide end position to the tele endposition. The zoom lens in the second example consists of a first lensgroup GR1 (positive), a second lens group GR2 (negative), a third lensgroup GR3 (positive), a fourth lens group GR4 (positive), and a fifthlens group GR5 (negative), which are arranged sequentially from theobject side. The first lens group GR1 consists of a negative lens G1, arectangular prism G2 to bend the optical axis through 90°, and apositive lens G3 having aspherical surfaces on both sides.

The second lens group GR2 consists of a negative lens G4, a negativelens G5, and a positive lens G6, which are cemented together. The thirdlens group GR3 is a positive lens G7 having aspherical surfaces on bothsides. The fourth lens group GR4 consists of a positive lens G8 havingaspherical surfaces on both sides and a negative lens G9. The fifth lensgroup G5 consists of a negative lens G10 and a positive lens G11.Incidentally, “LPF” denotes a filter, “CG” denotes a cover glass, and“IMG” denotes the receiving surface of the imaging element.

FIG. 3 is a diagram showing the lens arrangement of the zoom lens in thethird example. The arrows in the figure represent the loci along whichthe lens groups move in going from the wide end position to the tele endposition. The zoom lens in the third example consists of a first lensgroup GR1 (positive), a second lens group GR2 (negative), a third lensgroup GR3 (positive), a fourth lens group GR4 (positive), and a fifthlens group GR5 (negative), which are arranged sequentially from theobject side. The first lens group GR1 consists of a negative lens G1, arectangular prism G2 to bend the optical axis through 90°, and apositive lens G3 having aspherical surfaces on both sides.

The second lens group GR2 consists of a negative lens G4, a negativelens G5, and a positive lens G6, which are cemented together. The thirdlens group GR3 is a positive lens G7 having aspherical surfaces on bothsides. The fourth lens group GR4 consists of a positive lens G8 havingan aspherical surface at the object side and a negative lens G9, whichare cemented together. The fifth lens group GR5 consists of a negativelens G10 and a positive lens G11, which are cemented together, and apositive lens G12. Incidentally, “LPF” denotes a filter, “CG” denotes acover glass, and “IMG” denotes the receiving surface of the imagingelement.

FIG. 4 is a diagram showing the lens arrangement of the zoom lens in thefourth example. The arrows in the figure represent the loci along whichthe lens groups move in going from the wide end position to the tele endposition. The zoom lens in the fourth example consists of a first lensgroup GR1 (positive), a second lens group GR2 (negative), a third lensgroup GR3 (positive), a fourth lens group GR4 (positive), and a fifthlens group GR5 (negative), which are arranged sequentially from theobject side. The first lens group GR1 consists of a negative lens G1, arectangular prism G2 to bend the optical axis through 90°, and apositive lens G3 having aspherical surfaces on both sides.

The second lens group GR2 consists of a negative lens G4, a negativelens G5, and a positive lens G6, which are cemented together. The thirdlens group GR3 is a positive lens G7 having aspherical surfaces on bothsides. The fourth lens group GR4 consists of a positive lens G8 havingan aspherical surface at the object side and a negative lens G9, whichare cemented together. The fifth lens group GR5 consists of a negativelens G10 and a positive lens G11, which are cemented together, and apositive lens G12 having an aspherical surface at the object side.Incidentally, “LPF” denotes a filter, “CG” denotes a cover glass, and“IMG” denotes the receiving surface of the imaging element.

Tables 1 to 4 below show the specifications of the zoom lenses inExamples 1 to 4.

-   Table 1-   Table 2-   Table 3-   Table 4-   Symbols in the tables above mean as follows.-   F No.: F number-   F: focal length-   ω: half field angle-   R: radius of curvature-   d: distance from one lens surface to next-   nd: refractive index for d-line-   vd: Abbe's number-   ASP: aspherical surface    The shape of the aspherical surface is defined by the formula below.    $x = {\frac{y^{2} \cdot c^{2}}{1 + \sqrt{1 - {ɛ \cdot y^{2} \cdot c^{2}}}}\Sigma\quad{A^{i} \cdot Y^{i}}}$    where,-   x: distance from the vertex of the lens surface measured in the    optical axis-   y: height measured in the direction perpendicular to the optical    axis-   C: paraxial curvature measured at the lens vertex-   ε: conic constant-   A^(i): the i^(th) aspherical constant

Table 5 below shows the value of fa/fw in the inequality (1) given abovewhich is applicable to each of Examples 1 to 4. TABLE 5 Inequality (1)Example 1 Example 2 Example 3 Example 4 fa/fw 1.045 1.113 0.988 1.157

FIGS. 5A to 16C show aberrations observed in Examples. FIGS. 5A to 5Care diagrams of aberrations in Example 1, with the zoom lens beingadjusted to the end position of the short focal length.

FIGS. 6A to 6C are diagrams of aberrations in Example 1, with the zoomlens being adjusted to a position of the intermediate focal length.

FIGS. 7A to 7C are diagrams of aberrations in Example 1, with the zoomlens being adjusted to the end position of the long focal length.

FIGS. 8A to 8C are diagrams of aberrations in Example 2, with the zoomlens being adjusted to the end position of the short focal length.

FIGS. 9A to 9C are diagrams of aberrations in Example 2, with the zoomlens being adjusted to a position of the intermediate focal length.

FIGS. 10A to 10C are diagrams of aberrations in Example 2, with the zoomlens being adjusted to the end position of the long focal length.

FIGS. 11A to 11 C are diagrams of aberrations in Example 3, with thezoom lens being adjusted to the end position of the short focal length.

FIGS. 12A to 12C are diagrams of aberrations in Example 3, with the zoomlens being adjusted to a position of the intermediate focal length.

FIGS. 13A to 13C are diagrams of aberrations in Example 3, with the zoomlens being adjusted to the end position of the long focal length.

FIGS. 14A to 14C are diagrams of aberrations in Example 4, with the zoomlens being adjusted to the end position of the short focal length.

FIGS. 15A to 15C are diagrams of aberrations in Example 4, with the zoomlens being adjusted to a position of the intermediate focal length.FIGS. 16A to 16C are diagrams of aberrations in Example 4, with the zoomlens being adjusted to the end position of the long focal length.

In the diagram showing the spherical aberration, the ordinate representsthe ratio to the open F value and the abscissa represents the defocus,and the solid line, broken line, and chain line represent respectivelyspherical aberration due to d-line, c-line, and g-line. In the diagramshowing the astigmatism, the ordinate represents the image height andthe abscissa represents the focus, and the solid line and broken linerepresent respectively the sagittal image surface and the meridionalimage surface. In the diagram showing the distortion, the ordinaterepresents the image height and the abscissa represents the distortion(%).

The zoom lenses according to the first to fourth examples satisfy theinequality (1) as shown in Table 5. As shown in each diagram ofaberration in the Example, each aberration with the zoom lense beingadjusted to the wide end position, the intermediate position (betweenthe wide end position and the tele end position), and the tele endposition is properly corrected.

The foregoing description is about some preferred embodiments of thedisclosure of the invention and it is intended that the configurationsand structures of all matter shown as preferred embodiments shall beinterpreted as illustrative and not in a limiting sense.

Therefore, the present invention contributes to the improvement (inimage forming performance) and miniaturization of the zoom lens to beused for video cameras and digital still cameras.

INDUSTRIAL APPLICABILITY

The zoom lens pertaining to the present invention may be applicable notonly to imaging devices such as digital still cameras and digital videocameras but also to other imaging devices to be built into mobilephones, personal computers, and PDA (personal digital assistance).

1. A zoom lens of the type having a plurality of lens groups and varying in power in response to variation in intervals between the lens groups, which comprises a reflecting member to bend the optical axis passing through. the lens groups and a last lens group (counted from the object side) which is composed of a negative lens group and a positive lens group, with an air layer interposed between them (arranged sequentially from the object side).
 2. The zoom lens as defined in claim 1, wherein the lens groups are constructed such that the first lens group (counted from the object side) is stationary and contains said reflecting member.
 3. The zoom lens as defined in claim 1, wherein the lens groups are constructed such that last lens group (counted from the object side) has a negative refracting power.
 4. A zoom lens of the type having a plurality of lens groups and varying in power in response to variation in intervals between the lens groups, which comprises a last lens group (counted from the object side) which is composed of a negative lens group and a positive lens group, with an air layer interposed between them (arranged sequentially from the object side)
 5. The zoom lens as defined in claim 1, wherein the lens groups are composed of five lens groups.
 6. The zoom lens as defined in claim 4, wherein the lens groups are composed of five lens groups.
 7. The zoom lens as defined in claim 1, wherein the negative lens group of the last lens group satisfies the condition defined by the inequality (1) below. 0.9<|fa/fw|<1.25 where, fa denotes the focal length of the negative lens group in the last lens group, and fw denotes the focal length at its wide end.
 8. The zoom lens as defined in claim 4, wherein the negative lens group of the last lens group satisfies the condition defined by the inequality (1) below. 0.9<|fa/fw|<1.25 where, fa denotes the focal length of the negative lens group in the last lens group, and fw denotes the focal length at its wide end.
 9. An imaging device equipped with a zoom lens having a plurality of lens groups and varying in power in response to variation in intervals between the lens groups and also equipped with an imaging element to convert the optical images formed by said zoom lens into electrical signals, wherein said zoom lens comprises a reflecting member to bend the optical axis and a last lens group (counted from the object side) which is composed of a negative lens group and a positive lens group, with an air layer interposed between them (arranged sequentially from the object side).
 10. The imaging device as defined in claim 9, wherein the lens groups are constructed such that the first lens group (counted from the object side) is stationary and contains said reflecting member.
 11. The imaging device as defined in claim 9, wherein the lens groups are constructed such that last lens group (counted from the object side) has a negative refracting power.
 12. An imaging device equipped with a zoom lens having a plurality of lens groups and varying in power in response to variation in intervals between the lens groups and also equipped with an imaging element to convert the optical images formed by said zoom lens into electrical signals, wherein said zoom lens comprises a last lens group (counted from the object side) which is composed of a negative lens group and a positive lens group, with an air layer interposed between them (arranged sequentially from the object side)
 13. The imaging device as defined in claim 9, wherein the lens groups are composed of five lens groups.
 14. The imaging device as defined in claim 12, where-in the lens groups are composed of five lens groups.
 15. The imaging device as defined in claim 9, wherein the negative lens group of the last lens group satisfies the condition defined by the inequality (1) below. 0.9<|fa/fw|<1.25 where, fa denotes the focal length of the negative lens group in the last lens group, and fw denotes the focal length at its wide end.
 16. The imaging device as defined in claim 12, where-in the negative lens group of the last lens group satisfies the condition defined by the inequality (1) below. 0.9<|fa/fw|<1.25 where, fa denotes the focal length of the negative lens group in the last lens group, and fw denotes the focal length at its wide end. 